1. Field of the Invention
The present invention relates to a liquid developer for use in the development of an electrostatic latent image in the fields of electrophotography, electrostatic printing and the like.
2. Description of the Prior Art
Various liquid developers of the prior art which have been used for developing an electrostatic latent image are composed of pigment particles and an additive such as resins and the like, both of which are dispersed in a highly insulating carrier liquid having an electric resistivity not lower than 10.sup.9 ohm cm and a dielectric constant not higher than 3. In the carrier liquid, the pigment particles adsorb the resins and the like to form a toner. The additive, for example the resin, serves to control the polarity of the pigment particles and improve the dispersibility thereof in the carrier liquid and the fixing property of the toner.
The known liquid developers have been prepared for example, by kneading the pigment particles (such as) carbon black and the like) along with a polarity controlling agent, a dispersing agent and a fixing agent, for example, vegetable oil such as linseed oil, soybean oil and the like, and resins such as an alkyd resin, polystyrene, an acrylic acid type resin and the like to finely granulate the kneaded mixture, followed by dispersion of the granules into the carrier liquid such as an organic solvent, for example, a paraffin type hydrocarbon and the like. In the case of such a liquid developer, the additive other than the pigment particles is generally of a very excellent dissolving property in the carrier liquid, and therefore, it is liable to desorb and separate from the pigment particles during the use of the liquid developer and the storage thereof for a long period of time to thereby deteriorate the chargeability or polarity, dispersibility and fixing property of the toner. As a consequence of the foregoing, it becomes impossible to use the liquid developer in practice. This is one of the greatest defects of such a liquid developer. As the toner of the liquid developer prepared so as to eliminate such defects, there has been known, for example, a graft carbon type toner in which carbon black is directly and chemically bonded with the particular resins contributing to the polarity control and dispersibility of the corbon black to improve the bond therebetween. However, since the toner materials of such graft copolymer, that is, the pigment and resin are combined with each other by a chemical reaction, the respective kinds are strictly restricted within such a range that the chemical reaction is able to take place. Even when the reaction is performed under a constant condition, it is difficult to always form a product of constant quality. Due to these facts, unreacted materials and various undesired by-products may get mixed with the liquid developer to decrease the developing property of the toner and cause the deterioration phenomena thereof.
In addition, it has been known that natural rubber and synthetic rubber are used as an additive to the pigment, or as a material of the graft-copolymer. Generally, such natural rubber and synthetic rubber are dissolved in a solvent and kneaded along with the pigment, followed by dispersion of the kneaded mixture in the carrier liquid to coat the coloring particles with the natural rubber and synthetic rubber. In this case, however, there is such a defect that the developing property of the toner thus obtained is remarkably deteriorated by a considerable amount of elution of the resin and rubber constituting the toner particles in the carrier liquid, the desorption thereof from the toner particles, the flocculation of the toner particles and the loss of the balance of the toner composition due to the increase in the dissolving materials in the carrier liquid.